The invention relates to a connection piece for a level metering device working on the guided microwave principle, with said level metering device comprising a single-wire line. A connection piece of this type comprises an upper connection piece part and a connection piece wall disposed below said upper connection piece part, said upper connection piece part having a connection piece wall height. Said upper part of the connection piece is configured so as to allow for the attachment of a level metering device working on the guided microwave principle, said level metering device having a single electrical conductor or a single electric line.
Such connection pieces are in particular mounted in receptacles such as tanks, reservoirs for any kinds of filling products, open receptacles, etc. Connection pieces of the herein mentioned kind can, however, be used in floor or wall elements.
The invention further relates to a combination of a level metering device working on the guided microwave principle and having a single single-wire line for guiding the microwave to the filling product to be measured, and an interference-reducing attachment means such as a connection piece or a flange, on which the level metering device is mounted.
In this context, it is the question of a guided microwavexe2x80x94also called xe2x80x9cTime Domain Reflectometry (TDR)xe2x80x9dxe2x80x94when high-frequency microwave pulses are guided along an electrical conductor. Such an electrical conductor can be, for example, a wire, a steel cable, or also a metal rod. The electrical conductor extends up to the filling product. When impinging on the surface of the filling product, the microwave pulses are reflected. The transit time of the pulses is evaluated by the electronic unit integrated in the most cases in the level metering device, and is outputted as the filling level. By means of such level metering devices, the filling levels of all kinds of bulk goods but also of fluids and liquids can be measured. The advantage of these level metering devices resides in that the density, conductivity and dielectricity of the filling product do not impede the measurements. Even changing bulk good properties cannot influence the measured value. Thus, in particular, lime, cement, cereals, plastics pellets, flour and gravel can also be measured by means of level metering devices of the mentioned kind. The filling level is also reliably detected in filling products exhibiting a changing humidity or varying dielectricity. In particular, even high, tapered receptacles, in which contact-free measurement procedures often exhibit difficulties, can be measured by means of the mentioned level metering devices without any problem. One level metering device of this type is, for example, distributed under the designation Vegaflex. In this respect, it is to be noted that the term xe2x80x9clevel metering devicexe2x80x9d also includes devices, the evaluating electronic unit of which is not directly integrated into a housing of the level metering device but is accommodated at another location.
Level metering devices of the above-mentioned kind are often mounted in connection pieces of a receptacle containing a filling product. For receiving a level metering device of the above-described kind, such connection pieces are basically comprised of an upper connection piece part in the form of a flange plate having a central bore therethrough, and a connection piece wall disposed therebelow, such as a tube piece, for example. The connection piece wall may be configured cylindrical in its cross-section or also polygonal. In most cases, said connection pieces are made of metal, and the upper connection piece part and the connection piece wall are correspondingly welded together in most cases. Of course, different connection techniques can also be employed for these connection piece parts depending on the field of application. Even a one-part configuration could be considered.
When a level metering device working on the guided microwave principle, is mounted into a connection piece, then superpositions arise in the so-called echo profile curve. These superpositions depend in particular on the connection piece radius and can propagate over several meters beyond the end of the connection piece, i.e. the free edge of the connection piece wall. With very small or even with very large connection piece radii, no or only relatively small superpositions arise, but such interfering superpositions are frequent with the connection piece sizes present in the industrial plant engineering. The superpositions complicate or even render a detection of smaller echoes impossible. Thus, in particular directly at the end of the connection piece or in the close neighborhood thereof, echoes cannot be or can only be hardly detected. In the worst case, such superpositions can have the effect that a reasonable level metering cannot be effected.
For eliminating this problem, it is already known to use a so-called horn which surrounds the level metering device at least in the area of the connection piece. In this respect, reference is made, for example, to DE 44 04 745 A1. In the therein shown device for level metering, a transmitter and receiver means and a single conductor extending into the filling product to be measured is present. Said transmitter and receiver means is connected with the single conductor via a coaxial line. In this configuration, electromagnetic waves emitted by the transmitter are guided through the single conductor towards the filling product, are reflected by the surface thereof and detected by the receiver means. By means of a matching funnel made of electrically conductive material, attached to a mounting flange, an impedance matching between the coaxial line and the single conductor is achieved, thus reducing reflection during transmission of the measurement signal. The matching funnel hereby comprises a funnel-type contour expanding towards the surface of the filling product.
From EP 0 780 664 A2, a sensor device for level metering is likewise known, wherein the signal emitted by a transmitter means is guided up to the filling product via a conductive rod. In this case, the rod is connected to the receptacle via a system of several flanges which could be interpreted as connection pieces.
It is also known to provide two parallel lines instead of the single-wire line; in this case, however, it is the question of a level metering device working on the guided microwave principle comprising a two-wire line. It is moreover known to provide a coaxial line instead of the single-wire line.
All these known solutions require large expenditures and are expensive to produce and are also difficult to mount. Thus, it is necessary, in particular with the level metering device comprising two lines, to provide spacers in regular intervals so as to ensure that said two lines always exhibit exactly the same distance therebetween. These spacers, however, generate interfering reflections and hence measurement inaccuracies. It may also be the case that deposits settle between two lines, a fact which could lead to a failure or inaccuracies of the measurement. The same applies to the solution using a coaxial line. The coaxial line has further the problem of not being usable in case of bulk goods.
The invention is therefore based on the technical problem of providing a connection piece, which is simple and cost-efficient to produce and, nevertheless causes minor interfering effects when being used in combination with a level metering device comprising a single-wire line and working on the guided microwave principle.
The invention is further based on the technical problem of providing a combination of a level metering device comprising a single-wire line and working on the guided microwave principle, and an attachment means such as, for example, a connection piece, serving for attaching the level metering device on a receptacle, with said combination of level metering device and attachment means being supposed to cause less interferences than hitherto.
The first mentioned technical problem is solved according to a first aspect of the invention by a connection piece comprising an upper connection piece part, on which a level metering device working on the guided microwave principle can be mounted by means of a single electric line. Below said upper connection piece part, a connection piece wall is disposed having a connection piece wall height. At least one electrical conductor is present spaced apart from the single electric line of the level metering device to be mounted on the upper connection piece part, and extending from the upper connection piece part up to at least the 0.25-fold of the connection piece wall. It is particularly advantageous that the conductor extends to a maximum of the 2.5-fold of the connection piece wall.
According to a further aspect of the invention, a connection piece is provided configured for being combined with a level metering device working on the guided microwave principle, which is equipped with a single electric line for guiding microwave pulses to a surface of a filling product to be measured and for back-guiding reflected pulses, said connection piece comprising an upper connection piece part, in which the level metering device working on the guided microwave principle comprising a single electric line can be mounted, and a connection piece wall disposed below the upper connection piece part and having a connection piece wall height, and at least one electrical conductor being mounted on said connection piece, which electrical conductor being disposed at a distance apart from the single electric line of the level metering device to be mounted on the upper connection piece part, and extending from the upper connection piece part only up to the area, which is critical due to interfering reflections.
Such connection pieces reduce the superpositions hitherto caused by the connection piece itself. Therewith, echoes on the conductor for the microwave can henceforth also be detected in the proximate range or can at least be better detected than to date. For the first time, a two-wire line is modelled by the at least one additional conductor in the connection piece in the hitherto critical range, which two-wire line then verges into a one-wire line in the actual measurement zone. Therewith, the positive properties of the single-wire line are combined in the connection piece area with those of a two-wire line in an extremely advantageous manner. The enhanced insensitivity of the two-wire line with respect to interferences is here used for the area of the connection piece. Therewith, the previous positive properties of the level metering device comprising a single-wire line remain maintained in the actual measurement zone within the receptacle. It is further to be noted that the connection piece wall can be composed of one or more segments. It is not imperative that a continuous wall is formed. Each wall configuration showing an extension height from the connection piece, has to be included here.
Basically, it is also to be noted that one or more additional electrical conductors can be mounted in the connection piece. These additional electrical conductors can either be mounted in parallel or non-parallel to the single-wire line of the level metering device. In the latter case, their spacing from the single-wire line increases towards the free edge of the connection piece wall. Thereby ensues a slow impedance matching from the so-called input (start of the single-wire line at the level meter device) towards the open wave-guide. By the increasing, slowly rising impedance matching, impedance leaps which are disadvantageous for a measurement, are largely avoided and hence thereby causing interfering reflections, as well.
In particular with connection pieces made of metal, it is advantageous that the at least one electrical conductor is in contact with the connection piece wall. Thereby, the at least one electrical conductor is brought to the same electric potential as the connection piece, a fact which contributes to reducing the interference influences.
As already mentioned before, it is advantageous that the at least one additional electrical conductor comprises an increasing distance from the single-wire line of the level metering device towards the free edge of the connection piece wall. It is hereby still further advantageous that the electrical conductor is bent at least in a partial length area, whereby impedance leaps and thereby the interfering reflections caused can be prevented in a still better manner.
Initially, it has already been stated that for a reduction of the interfering influences of the connection piece, it is advantageous, according to the invention, that the conductor/s extend/s at least up to the 0.25-fold height of the connection piece height. Depending on the field of application, extensions of up to the 0.3-fold, 0.35-fold, 0.4-fold, . . . , 2.5-fold height of the connection piece wall can be suitable. Of course, extensions ranging in between the mentioned multiples are also possible, such as, for example, an extension of up to 0.31-fold or 0.36-fold of the mentioned height of the connection piece wall. Very good results have also been achieved when the end of the at least one additional electrical conductor is located in the area of the free edge of the connection piece wall. Furthermore, all intermediate heights are feasible, i.e. the conductor extends up to an arbitrary height lying in between the above-indicated values.
The initial distance between the at least one additional electrical conductor and the single-wire line of the level metering device can be set depending on the diameter and the height of the connection piece. With a connection piece diameter of 31 cm and a connection piece height of 33 cm, for example, a distance of 8 cm has proven to be extremely appropriate. Further appropriate distances can be assessed by simple tests.
An extremely simple and cost-efficient variant resides in configuring the at least one electrical conductor as a metal rod comprising a thread on one end, by means of which said metal rod is introduced in a corresponding thread bore on the upper connection piece part. Yet, it is also possible to provide any other kind of mounting, e.g. welding, soldering or clamp connection. The electrical conductor can also be configured as a wire or a cable, which in this case extends into the connection piece through corresponding mounts in the desired manner. Moreover, it is to be added that the cross-section of the at least one additional electrical conductor can be circular, square, rectangular or polygonal.
The second technical problem is solved by the combination of a level metering device working on the guided wave principle and being configured comprising a single-wire line, and an attachment means such as, for example, a connection piece, on which in turn at least one electrical conductor is mounted, which is disposed spaced apart from the single electric line of the level metering device and has a length by far shorter than the length of the single-wire line, hence only extends in particular up to the edge of the attachment means which only reaches up to the inner receptacle or only slightly beyond. Therewith, said additional conductor reaches in particular maximally up to the connection piece edge facing the inner receptacle or slightly beyond. That is, the at least one further electrical conductor does not extend into the actual measurement zone within the inner receptacle, but only into the area which is critical due to interfering reflections within the attachment space or the mounting connection piece. When the mounting connection piece has a connection piece wall height, it then extends in particular maximally up to the 2.5-fold connection piece wall height. The connection piece wall height has in particular to be measured as of the location where the single-wire line of the level metering device exits from the associated mounting in the level metering device.
Of course, such a combination of level metering device and attachment means can be modified correspondingly in accordance with the foregoing explanations as to the inventive connection piece, i.e. several conductors can also be present in parallel or non-parallel to the single-wire line in the exit zone. In particular, said several additional conductors can also be bent, particularly also so bent that the distance from the single-wire line of the level metering device gradually increases towards the filling product to be measured. The conductors can be attached in any possible way, and can in particular also be mounted through a thread in the attachment flange of the level metering device. Moreover, any possible kinds of cross-sections are of course imaginable, in particular circular cross-sections, but also polygonal cross-sections. Moreover, the diameters of the additional conductors can be adapted to the respective conditions.